1. Field of the Invention
This invention relates to the carbothermic production of aluminum from aliminum oxide in a carbon-containing material. It particularly relates to apparatuses including a reduction furnace wherein alumina and carbon are reacted by a carbothermic process to produce aluminum contaminated with 5-15% aluminum carbide.
2. Description of the Prior Art
Many attempts have been made to produce aluminum by a carbothermic process to replace the universally used electrolytic process. A carbothermic process has many potential advantages which are becoming increasingly important as energy costs continue to increase.
All of these efforts have failed because they have invariably produced a mixture of aluminum metal and aluminum carbide. When such a mixture of 10-20% carbide or more cools to about 1400.degree. C., the aluminum carbide forms a cellular structure that entraps liquid aluminum; thus the mixture becomes difficult to pour. In consequence, unless extremely high temperatures are maintained throughout all of the steps, process manipulations of the mixture, in order to purify it, become extremely difficult if not impossible.
U.S. Pat. No. 2,974,032 and U.S. Pat. No. 2,828,961 have described results that are typical of those to be expected from carbothermic reduction of a stoichiometric charge of alumina and carbon in a conventional electrically heated smelting furnace. The metal produced from the former process contains 21-37% Al.sub.4 C.sub.3 ; the metal produced by the latter process contains 20% Al.sub.4 C.sub.3. These processes are limited because reactive carbon and/or aluminum carbide is always present in contact with the metal that is produced and because time is available for the metal to react with the carbon and then to dissolve carbide up to its solubility limit.
U.S. Pat. No. 3,929,456 and U.S. Pat. No. 4,033,757 disclose methods for carbothermically producing aluminum containing less than 20% Al.sub.4 C.sub.3, i.e., 5-10%, which comprise striking an open arc intermittently to a portion of the surface of the charge to be reduced.
However, advances have now been made in the art, wherein aluminum that is contaminated with about 20% aluminum carbide can be treated so as to obtain aluminum of commercial purity. One such technique is described in co-pending application Ser. No. 7,986 now U.S. Pat. No. 4,216,010. This technique is adaptable to the production of aluminum containing less than 20% Al.sub.4 C.sub.3 (i.e., 10%). It comprises the step of contacting a product containing from 20-35% Al.sub.4 C.sub.3 with a melt rich in alumina in the absence of reactive carbon. Such purification techniques can impart commercial vitality to older carbothermic processes producing heavily contaminated aluminum. Thus it becomes worthwhile to locate the best existing prior art and to improve the effectiveness thereof.
In view of rapidly rising energy costs and regardless of the method that is employed to produce aluminum containing less than 20% Al.sub.4 C.sub.3, it is clear that measures must be taken to limit the energy lost to vaporized products, as one such improvement. Energy lost to vaporization depends on the amount of vapor produced in the reduction and decarbonization steps and also depends on the amount of vapor that is recovered in back reactions which release heat at times and places within the system where that heat released can be employed in pre-reduction reactions.
The methods of U.S. Pat. No. 2,829,961 and U.S. Pat. No. 2,974,032 involve conditions where vapor production is minimized but only with respect to 35-45% of the aluminum values entering the reduction zone; they do not solve the vapor recapture problem. The method of U.S. Pat. No. 4,033,757 teaches that about 20% of the aluminum is vaporized to produce a vapor having a composition of about 50 mole percent aluminum and 50 mole percent Al.sub.2 O. It further teaches that a portion of the aluminum is condensed directly on a surface of the charge and a portion reacts with CO to form Al.sub.2 OC which then reacts with more CO to form Al.sub.4 O.sub.4 C, and this further reacts with aluminum carbide in the charge to produce aluminum liquid which flows to the hearth pool over unreacted charge. However, the disclosure points out that the capacity of the charge column to absorb heat from the back reactions of the vaporization products is not unlimited, and when vapor product exceeds the capacity of the charge column to absorb heat, it becomes impossible to keep the heated reaction zone down below the electrode system where it belongs. The result of this situation is that unreacted vapors break through the surface of the charge column and cause what are known as "blow holes".
Supplying power economically requires that AC power be delivered through electrodes at high voltage and low currents. Such high voltages are obtained when electrical current flows via open arc between electrodes and the charge to be reduced for producing aluminum. However, it has been found that such open arcs cause excessive vapor production, including vaporized aluminum.
It is also known that high voltage AC heating is possible with electrodes which are submerged in and are conducting through the charge to be reduced. However, such a heating arrangement prevents the formation of an aluminum product containing less than 20% carbide when the metal product is held in contact with the semi-reduced charge layer through which the heating current is conducted to a hearth.
There is accordingly a need for a carbothermic reduction process wherein vapor production is minimized, AC current is used, control is simplified, and the metal product is quickly removed from contact with reactive carbon.